1. Field of the Invention
This invention relates to a focal plane shutter for cameras which is provided with two shutter blade groups including a plurality of blades.
A blade room for a second blade group is formed between a shutter base plate and a middle plate and a blade room for a first blade group is formed between the middle plate and an auxiliary base plate, by separating a space between the shutter base plate and the auxiliary base plate by the middle plate. A push pin which is planted on the shutter base plate pushes a portion on the lower area of the middle plate toward the auxiliary base plate so that the middle plate bends in the shape of an arc. Also, the area of the middle plate below the exposure aperture has the narrowest width in the portion which is pushed by the push pin and the pushed portion of the middle plate has a shape by which the area of the middle plate below the exposure aperture is easy to bend. As a result, when part of a slit-forming blade of the second blade group moves to the lower side of the exposure aperture in exposure operation, the slit-forming blade does not strike against an edge of an opening portion for the optical path of light from an object on the middle plate even though the slit-forming blade bends.
2. Description of the Related Art
As known publically, focal plane shutters for cameras each of which is provided with two shutter blade groups each having a plurality of blades are formed in such a way that two blade rooms are formed by separating a shutter base plate from an auxiliary base plate by a middle plate and the two shutter blade groups are arranged in these blade rooms respectively. And, these three plates are provided with openings for guiding light from an object respectively in such a way that the openings overlap with one another. And, one or more of these openings form an exposure aperture which is shaped like a rectangle.
Also, each of the two shutter blade groups is composed of: two arms one ends of which are pivotably fitted to the shutter base plate individually on an area on one lateral side of the exposure aperture, or on an area exterior to one short side of the rectangle forming the exposure aperture; and a plurality of elongate oblong card-shaped blades which are pivotably supported by both these arms with these blades arranged toward the free ends of these arms in turn. And, when the arms are rotated in shooting, each of the blades moves while the straight end edge of each of the blades running in the direction of the width of each of the blades is being kept parallel to a long side of the exposure aperture. And, when these shutter blade groups separate from an exposure aperture, the blades of each of the shutter blade groups are folded while amounts of overlaps between the blades are becoming large. When the shutter blade groups cover the exposure aperture, the blades of each of the shutter blade groups are unfolded while amounts of overlaps between the blades are becoming small.
Also, when the two shutter blade groups perform exposure operation, one shutter blade group which is unfolded (and which is called “first blade group” hereinafter) first begins to operate and is opening the exposure aperture while amounts of overlaps between the blades of the one shutter blade group are increasing, and the other shutter blade group which is folded (and which is called “second blade group” hereinafter) subsequently begins to operate in the same direction as the first blade group does and the other shutter blade group is covering the exposure aperture while amounts of overlaps between the blades of the other shutter blade group are decreasing, so that an image plane is continuously exposed from one side to the other side by a slit that is formed between a blade running at the tail of the first blade group (and called “slit-forming blade” hereinafter) and a blade running at the head of the second blade group (and called “slit-forming blade” hereinafter). In this case, it is publicly known that a spring or a motor is used as a driving source for performing the exposure operation.
And, in the case where a spring is used as a driving source, focal plane shutters for cameras are usually formed in such a way that: a driving member which connects a driving pin to one of the arms of the first blade group (and which is called “first blade group-driving member” hereinafter) is rotated by the biasing force of a driving spring (which is called “driving spring for the first blade group” hereinafter); and a driving member which connects a driving pin to one of the arms of the second blade group (and which is called “second blade group-driving member” hereinafter) is rotated by the biasing force of a driving spring (which is called “driving spring for the second blade group” hereinafter). As known publically, focal plane shutters having such a constitution include focal plane shutters which are called “direct-type focal plane shutter” and “locking-type focal plane shutter” respectively according to a difference in constitution for keeping the first blade group-driving member and the second blade group-driving member in their states just before the start of exposure operation.
Also, it is known that, among such focal plane shutters in which exposure operation is performed with a spring as a driving source, there are focal plan shutters of both direct type and locking type in which: not only the second blade group, but also the first blade group is made to become folded before shooting so that the exposure aperture is fully opened and an object to be shot can be observed on a monitor through an imaging sensor for shooting; and when a release button is pressed in shooting, exposure operation is performed in the above-described manner after the first blade group is made to change from the state of the first blade group being folded to the state of the first blade group being unfolded in order to close the exposure aperture. And, Japanese Patent TOKUKAI No. 2003-222928 discloses a direct-type focal plane shutter having such a constitution.
On the other hand, it is known that, among focal plane shutters for which motors are used as a driving source, there are focal plane shutters in which a driving pin integrated with a rotator of a first motor is connected to one of arms of a first blade group and a driving pin integrated with a rotator of a second motor is connected to one of arms of a second blade group. Japanese Patent TOKUKAI No. 2004-264468 discloses a focal plane shutter having such a constitution as the second embodiment.
And, as in the above-described focal plane shutters in which exposure operation is performed using a spring as a driving source, the focal plane shutters having such a constitution can be also made to work in such a way that: the focal plane shutters perform cocking operation immediately after shooting; and the first blade group is worked to be folded from a state in which the first blade group is unfolded, immediately in shooting, and the second blade group is subsequently worked to be unfolded from a state in which the second blade group is folded. However, as in the focal plane shutter disclosed in Japanese Patent TOKUKAI No. 2003-222928, focal plane shutters can be also made to work in such a way that: both of the first and second blade groups are folded before shooting so that the exposure aperture is fully opened and an object to be shot can be observed on a monitor through an imaging sensor for shooting; the first blade group is unfolded to close the exposure aperture by pressing a release button in shooting; and then, the first blade group is first made to start to perform its exposure operation and the second blade group is next made to start to perform its exposure operation.
As described above, in focal plane shutters each of which is provided with two shutter blade groups, there is necessity that two shutter blade groups should be made to operate in various manners different from each other in accordance with specifications for cameras to be adopted. Accordingly, diving devices for driving the two shutter blade groups respectively are different from each other, in accordance with the specifications for the two shutter blade groups. However, the two shutter blade groups themselves have approximately the same fundamental constitution and approximately the same configuration. And, there is necessity that such two shutter blade groups should operate stably at high speed not only in exposure operation but also in another operation different from exposure operation.
However, each of the shutter blade groups is composed of two arms and a plurality of blades shaped like elongate oblong card and pivotably supported by the arms, as described above. In addition, each of the blades is made of a thin material in order to make the blades have light weight. As a result, when a shutter blade group is worked to change from the folded state of the shutter blade group to the unfolded state of the shutter blade group and is covering the exposure aperture, there is no means supporting the middle portion of each of the shutter blade groups in their longitudinal directions, so that each of the blades inevitably bends while the blades are running. As a result, the middle portion of each of the shutter blade groups may inevitably bend in the direction perpendicular to the surface of each of the blades. And, when the exposure aperture is closed, a bend that is formed on the approximately middle portion of the slit-forming blade moving at the head of the blades in its longitudinal direction inevitably strikes against an edge of the opening for the optical path of light from an object, the opening being provided with the middle plate. As a result, a situation where operation of the shutter blade group is disturbed (the occurrence of bounces of or tilts of the shutter blade group) or a situation where the blades are damaged inevitably occurs.
In order to prevent the occurrence of such situations, Japanese Utility Model registration No. 2589062 discloses countermeasures in which both edges of the opening formed on the middle plate and for guiding light from an object, which correspond to the two long sides of the exposure aperture, are formed to be approximately shaped like the letter “V”, or in addition to this matter, a groove having a predetermined length and running from the apex of the V-shaped edge in a direction of the blades of one shutter blade group moving is provided for the edge toward which the one shutter blade group moves while its blades are being unfolded in exposure operation.
However, further improvements in a plurality of blades for moving a shutter blade group at a high speed have made the method as disclosed in Japanese Utility Model registration No. 2589062 insufficient for some focal plane shutters for cameras, in recent years, Japanese Utility Model registration No. 2589062 disclosing the formation of the V-shaped edges of the opening for guiding light from an object on the middle plate and the formation of a groove in the apex of the V-shaped edge. Accordingly, it has been also considered to form a focal plane shutter in such a way that the middle plate made in the shape of a plane plate is not fitted to the shutter base plate with the middle plate having a plane plate-like shape unlike conventional manners but is fitted to the shutter base plate while an area toward which the shutter blade groups move with a plurality of their blades unfolded in exposure operation originally is becoming an arc-shaped surface swelling slightly in a direction in which the blades swell.